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ALCOA 




ALUMINUM COMPANY OF AMERICA 

PITTSBURGH, PENNSYLVANIA 







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COPYRIGHT, 1938 



ALUMINUM COMPANY 



OF AMERICA 






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CONTENTS 



Page 

Characteristics of Aluminum 7 



6 

Polishing 



I I 



\1 ECHANICAL FINISHES 9 

Grinding 10 

11 

o • • • 
Roughing 

Greasing 1- 

Buffing 13 

Typical Polishing Procedures 16 

Coloring 1" 

Finishing Sequences 17 

High-Lighted Finish 18 

Scratchbrush Finish 19 

Satin Finish 21 

Hammered Finish 22 

Fluting 22 

Sandblasting 23 

Tumbling and Burnishing 25 



n u**v« m^ v^*. *x*u*««*~ 



Chemical Finishes 29 

Frosted Finish 29 

DifFuse Reflector Finish 31 

Etching 31 

Practices for Applying Caustic Etched Finishes 32 

Alrok Finishes 33 

Electrolytic Oxide Finishes 35 

Alumilite Finish 35 

Characteristics of the Finish Jo 

Color Matching of Alumilite Finish 41 

The ( ioating Process 43 

Alzak Hetleclors 45 



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CONTENTS— Concluded 






Electroplating on Vlumini m 



Zinc Plating 



Chromium Applied Directly on Uuminum 
Nickel Plating 

I' «<lnfc Kmployed l<»r Applying the Nickel 
( H hei \l«ijih <»n Uuminum 
Black Nickel 

Test s of 1 )eposi1 s 

\lU.\I) PilODl (IS 



hiM. Lacqi er \m> Enamil I imshes 

b£ffec( <>f \lli»> ( lomposil ion . . 



Surface Preparat i<>n 
\lri bods <»l Vpplica! ion 
Priming Paints 



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Finishing < <>aK 
( Hear F inishes 
\ itreous Knann I 



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INTRODUCTION 



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HIS BOOKLET has been prepared to assist 

the users of aluminum in solving their finish- 
ing problems. The procedures described have 
given satisfactory results in practical plant op- 
erations. Under different conditions, however, 
the directions may have to be modified. They 
are presented, therefore, as a guide for finishing 
procedures. They should be tested and perhaps 
modified before they are adopted for any given 



| application. 



Many attractive finishes may be applied to 
aluminum. To secure the best results, accurate 
knowledge of the characteristics of the finishes 
and the applications for which they are best 
suited is necessary. In an effort to indicate the 
held of usefulness of the various classes of fin- 
ishes, the valuable features, the limitations and 
the methods of the application of each have been 
pointed out. 

Aluminum Company of America maintains a 
technical staff whose services are available to 
users of aluminum for advice on all phases of 
finishing work. 



CHARACTERISTICS OF ALUMINUM 



I 



• 




LUMINUM owes its wide and ever-growing use to certain 

well-known characteristics. Light weight, more than any 

other property, accounts for its amazingly rapid commercial 

I de\ ^'lopment. Its pleasing color dulls but slightly with use, and 

I is not greatly marred by oxidation. Its resistance to corrosion is 

sufficient for many applications with no additional protection. 
Other chara< l eristics are its high electrical and thermal conduc- 
tivity, its high reflectivity for light and thermal radiation. 

Commercially pure aluminum is soft, ductile, and can be fabri- 
cated into complex shapes. Its physical properties may be altered 
(1) by cold working to increase tensile strength and hardness, 
I (2) by alloying with small percentages of other elements to produce 

a wide range of properties, and (3) by heat treatment of certain 
' of its alloys to add to their strength and hardness. The casting 

I alloys can be cast in sand, dies or permanent molds. Wrought 



aluminum and its alloys are obtainable as plate and sheet; bar, 
rod, wire and rivets; seamless tubing and pipe; molding and 

I stru< lural shapes; screw-machine products; stampings, forgings 

| and pressings; and impact extrusions. 

For many purposes, standard sheet as it comes from the mill 
may be employed without any special finishing operation. Where 
appearance is not a factor or where the struct uxe will be painted, 
"gray plate" may be used. 

If the normal characteristics of plain aluminum do not provide 
the desired qualities for some specific application, there are avail- 
able a great number of finishes to alter these properties. There 
are finishing processes that enhance the appearance of the alu- 
minum surface, that increase its resistance to corrosion and 
abrasion, and that improve its reflectivity. 



MECHANICAL FINISHES 




* 



LUMINUM in its natural form has a pleasing appearance. 
bu1 l he metal surface may be improved for some applica- 
tions by such mechanical processes as polishing, buffing, hammer- 
in-, sandblasting, satin finishing or burnishing. These finishes 
may serve the purpose of blending the metal into an appropriate 
decorative scheme, or may simply form the foundation for other 
decorative <>r protective finishes. 

\s in the case of most processes, the quality of the finish will 
depend largely on the skill of the operator. However, the shape 
ol I he metal article and the nature of the alloy will also be de- 
lermining factors. Hence, the actual plant practices prescribed 
here may have to be modified somewhat to accommodate any 

particular alloy or shape. 

In order to avoid ambiguity the various operations of me- 
chanical surface finishing are classified and defined according to 
the system used in general by the trade. 

1. Grinding: Operations involving the use of a bonded abra- 



sive \\ 



heel 



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2. Polishing: Those operations involving the use of "set-up" 
wheels, belts or discs. Abrasive is attached to the periphery of 
the wheel or to the surface of the belt or disc, by means of glue. 
The polishing operations are further subdivided as follows: 

Roughing: Coarse abrasives (60-100 mesh). No lubricant 

Used. 

Greasing or Oiling: Finer abrasives (120-400 mesh). Lu- 
bricant is used. The greasing operations are sometimes further 
divided and known as greasing, grease fining, grease finish 
and grease coloring, depending upon fineness of abrasive. 

9 




10 



A LUMIN1 M niMPANY OF AMERICA 



3. Buffinii: The expression "cutting down is sometimes 
used. In this procedure abrasive is applied to a sewed muslin 
wheel from a cake compounded of tripoli and other ftbn m 
v\ it 1 1 a binder which supplies the lubricant. 



/. Coloring: Sometimes termed "color buffing This pro- 
cedure differs from the above in thai unstitched muslin wheels 



,ti e m< -d. and I he abrash e is finer. 



\\\ means of these mechanical finishes, I h- surface is freed from 
in uiarities or scratches produced during < -tin oi fabrication, 
and, if desi d, is brought t< > a high luster. 

'I I i mechanical «<ruipment, both hand and automatic . foi us 
on aluminum is of the same ;neral type as that employed for 
i met In ii- lection, certain fundamentals should be 
rved. 1 equipment must be of ample capacity, and sturdily 

• structed to withstand beav) and continuous rvic< !i 
should be set up on a lid foundation to eliminate vibration 
Polishn h< < l> must run true and must be properl) balanced. 
"I h( ■ omp on and i onstruction of 1 1 !<•-<• wheels and theit d 

r<> v ;n i. it - i hat ai i di mussed lal ei 



<,|{|\I>IM, 



i.ar _< astin^s, w ln< 1 1 cannot !>«■ held against an ordinal \ wheel, 
<l< m d special methods o( handling to prepare them foi I .1 
finishii procedure- Portable, rotarj air ^rrindei d( elopin 
10(1 f p.m., and set up with cup-grinding wheel I I 

I lk'mj: down tin surl I la- i rou^h grinding 

>ns BTi Q< foi prep aslinp; for andbl 

j uiarities, if not removed in the initial treatment, \n 



otni more pro mi i) and m< ii < 



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rations. Koi flat surf ind particularly foi removing 

; ' mi < »f met al pi nolic -r< >nded ili nn-cai 

with \< JJ to \. 



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MECHANICAL FINISHES 11 



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Rough grinding is preferably done dry but tallow or lard oil, 
if applied with care, may be used when necessary. In cases where 
oil and grease are present in excess, they can be removed by 
preheating the casting to a temperature of 400°F. to 600°F. for 
approximately one-half hour.* 

Detrimental depressions in the surface of a casting, resulting 
from sand holes and porosity, may require welding. The welded 
areas can be blended into the surface of the casting by means of 



I a cup wheel or by means of a disc sander as described under 



"Roughing." Welding may cause discoloration in castings thai 
are oxide coated by the Alumilite** process. This can be min- 



| imized by healing the casting to between 900°F. and 960°F. for 



1 hour to 5 hours prior to sandblasting and finishing by the 
Alumilite process. 



| POLISHING 

| Roughing: The coarse polishing operation, known as rough- 

ing, is employed as a preliminary step in polishing very uneven 
or deeply scratched surfaces. Sand castings always receive this 

| treatment. Portable equipment such as described on page 10, 

1 1 1 1 ( I er ' ' < « rinding, ' ' is sometimes used on large cast i ngs. Instead of a 

' cup-grinding wheel, flexible carborundum paper discs are used. 

| These discs are about 9 inches in diameter and vary in coarseness 

of abrasive, 21 mesh to 80 mesh, depending upon the nature of 

■ the work. 

} Die castings and fabricated articles do not necessarily require 

the roughing operation unless their surfaces are unusually rough 

I or marked. 

1 For roughing, a wheel made of pieces of muslin or canvas glued 

or cemented together is needed. Other materials, such as wood 

■■ faced with leather, sheepskin, felt or flannel can be used. In gen- 

eral, the weight and grade of the material and the method of 
joining I he pieces influence the wheel's flexibility which is im- 
portant in the polishing of curved surfaces. 

*This method cannot be applied in the case of heat-treated castings because 
of the adverse effect on their mechanical properties and dimensions. 
**Patented process for producing oxide finishes. 



12 



\ L l \] I \ I \1 COMPANY OK \ M E KH \ 



Regardless of material chosen for the wheel, tin- facing pro- 
lure i> the same. \ layer <>l glue is applied, in which No. <>o 
, \«> loo emer> or fused aluminum oxide abrasive is embedded. 
Careful aj > | > I i cation of plue and abrasive will prevent man> 
polishing troubi< - liolh the composition and application of ih« v 
glue should be determined b> up-to-date recommendations <>l 
abrasive sprain suppliers and manufacturers 

The diameter o! the wheel ma) himl from <> inches to 20 
Inch and i i > thickness from LJ^ inches to 3J^ incl lepend- 
on Ui »f 1 1 1 < - work to be Qnished. I h<' wheel should have 

a periphei il spei I oi about 6000 feet per minuti Slowei speeds 
tend to tear the abrasive >utofth glue wliih* higher sp< Is cam 
. \. . beati - which ma) break down th<- glue \ lubricanl 

su( li ••- tall mixture <<l tallow and hud <»il ma> In I on 

th \\ I reduce the heating I hi- must l>< d ae with can 

how . . . i in ■ |( i 1 1 tvoid driving the lubricanl into i Ik- p"i of 

(id »pottii h surface b> epa in some latei finish 

01 I <M. 



i 



Gretutitifz: I inishin itmenl ometime known as ml 

refi ment i the hing procedure ■ 1 siinil i • if 

• p{ that a hdirii ml i always used on t h« ■ wheel Sand 
ii. ii triabl) • n iln- greasing treatment al i I 
i I h< astii in iv< n ii ;«- the Hi ; polishing >>\> lion 
I al ated artii h that have be< n roughed on .« i a^ ui can 
•I require il i preliminary operation before bufli l« 
i ■■ > i ■ " . |K - •• ■ a felt wheel, < metim< a si kin uhecl 

I with IS 100 to N< - (| ry is u I Hi \\* 

■< d .i In ish e i ban i quired fi >i i oughi I h< fel 

has i h< tdvant I uniform density and cont n 

d nf <d Sei< i i u »n i »( t he pi < >pei ide «>l \> 

I \ < n l< -in-, e v er al d < rei ' h ard n < bh 

I pl< i pi bh _•! »rj( m ■ life foi hm h 

lai \\ h l-| rid dimension < mu 

a> 1 h I ' r mj-hifi tid an on n 



MECHANICAL FINISHES 13 



siderations. The lubricant is important as a means of reducing 
the danger of burning, and may be tallow, oil, beeswax or tallow 
i ompositions. A single wheel greasing operation is generally 
sufficient. A higher polish may be obtained, however, by the use 
of a ^ heel faced with No. 100 emery followed by treatment with 
a second wheel faced with No. 220 emery. 



BUFFING 



Buffing is the next finishing operation used on an aluminum 
surface when the objective is to bring out the high luster of the 
metal It differs from roughing and greasing in that the abrasive 
is embedded in a grease binder and rubbed on instead of being 
glued to the face of the wheel. The selection and proper applica- 
tion of the most suitable abrasive for the work is important. 

Tripoli powder is by far the most common of the buffing agents. 
Chemically it is high in silica and its value as an abrasive lies 
chiefly in its physical structure. The grains are soft and spongy, 
and free from sharp crystalline cutting edges. They crush down 
in buffing to present fresh polishing surfaces. Tripoli powder is 
available in several degrees of fineness. Fine-grained material is 
preferred in that it produces a brighter finish. Purity and uni- 
formity of grain size are also essential to secure satisfactory work. 

The abrasive powder is mixed with a grease base and molded 
in a cake. Cakes in which tripoli powders of various degrees of 
fineness are incorporated may be purchased from suppliers. The 
abrasive cake is pushed against the wheel as it revolves. The heat 
generated softens the binder and the abrasive becomes attached 
to the w heel. This procedure must be repeated frequently to pre- 
serve the cutting action. 

The wheels are run at a higher speed than that used in the 
earlier finishing operations in order to hold the edges of the 
bulling wheel close to the work. It is possible to increase the 
speed of the wheel because of the decrease in heating effects. 
Buffing wheels should have a peripheral speed of 7000 to 7500 
feet per minute. 




Jewelry made of alumi- 
num may be finished by 
buffing, ball burnishing 
or hammering. 



and-casl aim rtum 
spandrels f\ shed i 

sa ndblas t in g , dep la I in g , 
and oxidized high light- 
ing- 



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MECHANICAL FINISHES 15 



Difficulties Encountered in Buffing: The difficulties fre- 
quently encountered in buffing are the formation of pits, surface 
roughness, and buffing clouds. These difficulties may be cor- 
rected by attention to one or more of the following factors: 

Hardness of buff 
Buffing compound 
Peripheral speed 
Pressure against buff 

The harder the buff the better the cutting qualities which will 
produce a smooth surface. However, there is greater tendency 
to form pits or cloudy or streaked areas. Hardness in a buff re- 
sult from: 

Material of high thread count 

Close spirals in stitching and short stitches 

Few or no -pacers between discs, and few or no spacers between sections 

\ liL'li [><-riplii'r<i| ^pn.'d 

The softest buffs, such as used in coloring, are made from ma- 
terial having low thread count. The only stitching is around the 
arbor hole — just enough to hold the sections together. As many 
as four spacers may be used between discs, and between sections. 
The spacers referred to are generally 3-inch to 4-inch discs, and 
may be of paper or cut from worn buffs. 

Where pitting and clouding difficulties exist, attention should 
be directed toward softening of the buff. The type of buff, in 
which all the cloth is exposed to the cutting surface on a bias, 

provides a soft buff. 

The buffing compound should be selected for the particular 
job on the basis of test. Abrasives used in buffing compounds 
vary considerably in hardness and cutting properties. The grease 
binders are also varied, depending upon melting point desired. 
With decrease in the melting point of the binder there is less 
danger of pitting and overheating, and a decrease in luster, but 
the cutting quality and tendency toward cloudiness are increased. 
A greater amount of lubricant is needed for buffing aluminum 
than for harder metals. 



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High peripheral speeds inerease efl'ei live buff hardness and im- 
pro> e < ui tin <|nalii n->. Imi increase the tendency for pit ting and 

Pol ni.it lOH of clc n i c I \ arras. 

High }>(« ure againsl the bull" increases the cutting rate, bui 

also increa - 1 h«- tendency for pilling ami cloud formal ion . In 

eneral pre urea used in polishing or buffing aluminum or it 
.ill .n « [eg t han i hose used for harder metals. 

Vll of th< i toi ire important, but in addition thereto, th< 
operating technique gained i h rough experience, observ a lion and 
instru< i ion is an e < m ial requirement . 

< OLORING 



The final operation of I ho I i rushing procedure is known a 

luiiii. Vctuall) iIk- metal surface does noi ehar \ uoticeabl) 
in coloi bui lakes on a characteristic luster md high glo 
Preparation b> cleaning in benzine and drying in sawdust pre- 
cedes coloring li is frequently found desirable in preparing a 
surface lor finishing b} the Mumilite pro<< io -i\e the article 

light i in ie etch between the bulling md coloring operation 
Id chemical treatment removes embedded abra ive which, if 
allowed to remain, tends t<> produce surface discoloration during 
trolyti* oxidation. The coloring is done <»n open muslin <>i 
llannel wheels, similar to the buffers. The} m«\ have spacei 
between them to make them softer, and a periphei d sp d of 

>00 to 8000 feel per minute is used. The abrasive is fl silk t 
mi o d w it h a grease base. 

FINISHING SEQI ENCES 

This discussion <<l finishing materials and procedures A\ 
general dn tions for surfacing aluminum. r \ o finish an\ par- 
tis i Mar aluminum article, however, the spe< ifi< details of the pi 

ess must be worked out experimentally, bringing int nsidera- 

t i. >n the nature of the metal, the surface md the desired finish. 
I he table <»n page 16 indicates some of the variations in method 
required to produce the same kind of finish on different forms 

ilummum. 




1 



ALUMINUM COMPANY O F h M ERIC \ 



HIGH-LIGHTED FINISH 

\ ,.i \ attractive effects ma> l>c obtained on aluminum artich 
n the process known as high Lightinj \\hi<h the raised 

portions oi the surface one type of finish, whil the recesses n 
ceiv< another. Such decorative treatment has been widely en 
ployed on refrigeratoi evaporator doon V\n background i nd- 
blasted and the high-lighted areas buffed b> \\u> following *>< 
• 1 1 j i pei i t ions . 



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I | • protected by rm covering i b au ad • ia|n 

,1,1 t I paint while tin- background I • I . * - 1 * . i Vr 

Saudi i 

I i I |, [ h H i h ir- it ! U prepan tin IblaNti 

; , «« \ lini Iiiii^' I.n I Ik Mniinllti |u i n i 



I h. I . . by t uping i u painl in bill llu 1 In^'h lighl 

\ \ i , \ Ium u hi « |j 1 1 i i 'I 1 '" final opera! ii I lit**'! i»« i n 

!KntI\ 






It : i , Iiiimjo. t high 1 1 tint! cast in j lias been worked out 
i<l< ■ |,|. <i< til foi ai « hit<'< i ui al appl < »< 

I i is i l'\ ni' .hi- of r "I a1 M w In I i ii 

im. m<! I- »i flat ai I ii It bell saiuh I used 

\N i,. i i < ill- landard 6 rich to 12-inch d imi n 

hj. d 1 1|. i « ive t li<- i |ui 1 i hickm - and I ed 

Ii \ \ i I crnei y. I h< n< u mal pi ul in 1*1 1 m 

i win 'I -Ii Id h< <<h i«l I It' rllic -Im 'lil'l l» I 






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in III. UM*d, t h< l>« i < «lu I 

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im*d all \\ ti< < K vn il h \ I 

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MECHANICAL FINISHES 19 



I 



muslin or felt wheel and a compound such as tripoli. A buffed 
Mirface may be given a still higher luster by colorin 

Where plain flat surfaces must be high lighted absolutely 
smooth and free from ripples, the belt sander is used. Cloth 
belts of Nos. 80, 120, 140 and 180 emery are employed in the 
order given, depending on the texture desired. No. "0" tallow 
and a small amount of machine oil are required as a cutting lu- 
bricant, or a thin paste of paraffin and turpentine also makes a 
satisfactory compound. A belt-sanded surface may also be buffed 
with muslin wheels and tripoli if a lustrous finish is desired. 



SCRATCHBRUSH FINISH 



A coarse-lined finish may be obtained by the application of a 
rotating wire brush to the aluminum surface. Fineness of the 
finish may be regulated by the size of wires used in the brush. 
Wire wheels 10 inches in diameter, made up of wires 0.015 
inch in diameter, revolving about 2000 r.p.m., are generally satis- 
fa( lory. Wires may be composed of brass, stainless steel, nickel 
or German silver. 

Dirt and grease must first be removed from the aluminum 
surface by rubbing it with air-slacked lime. Then it is held lightly 
against the edge of the revolving wire brush until the desired 

I roughening is attained. On castings, the scratchbrush finish is 

applied, preferably after a fine sandblast, but if the surface is 
exceptionally rough, it may be necessary to use the medium 

| sand. The melhod of application for castings is identical with 

the procedure for other forms of aluminum with the exception 

' of wheel speed, which is reduced to 450 r.p.m. to 600 r.p.m. 

| for best results. Higher wheel speeds result in excessive tearing 

of the metal surface and subsequent lack of uniformity in the 



tinal finish. As the wheels are used, they gradually accumulate 



| oxide and metallic aluminum particles and must be cleaned fre- 

quently. This can be done by using a pumice stone or soft brick. 
' As the wheel wears, the wires also become bent and dull, necessi- 

| taling frequent reversal of the direction of rotation. 




i 



A combination of hammering ami 
aid work produces the rich, ar- 
tistic finish of these aluminum 

decorative piers. 



1 1 
I 



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Aluminum king utensils n 
ceive On brilliant luster by poi 
i. The pro 8 inclu 
<il oj}( rations, all o t hich arc 

descriit'd in litis chapter. 



( in w wire bi t(j is a 
n u nt means oj n l\ ing ///< o- 
oton ) oj large pla m a I hi 

meckan il dm apj i here 

to lit* < nds oj vtoragi lanl 







MECHANICAL FINISHES 21 



SATIN FINISH 

Satin finish is a modification of the scratchbrush finish ob- 
tained by treatment of the melal with a finer wire-brush wheel 
or by abrasives, ll imparts a soft, smooth sheen with lower re- 
flectivity lhan a highly polished surface. The soft effect results 
from the tiny parallel lines scratched on the metal. Various ef- 
fects may be obtained by changing the angle of contact of the 
surface with the wire brush. Wires of 0.002 inch to 0.005 inch 
in diameter are most often used; the finer the wire, the finer the 
texture of the finish. Some difficulty may be experienced in ap- 
plying l liis linish uniformly to large areas. 

For some surfaces, like the inside of cooking utensils, the same 
kind of satin finish may be obtained without the use of the wire 
brush. The object may be rotated in a chuck while holding an 
oily abrasive cloth firmly against the surface as it revolves. A 
further rubbing with steel wool dipped in oil and emery powder 
and a final cleaning with a rag while the article is still revolving 
gives a brighl silvery finish. Steel wool lubricated with soapy 
water or greaseless polishing compounds can also be used to 
at in finish aluminum very satisfactorily. 

To produce a satin finish on castings, their surfaces must first 
!x polished as described under high lighting. After the use of 
the No. 180 emery, the surfaces are buffed with muslin or fell 
wheels with greaseless polishing compound. Various grades of 
greaseless compounds give a coarser or finer finish, depending 
upon the grain size of the abrasive contained in the compounds. 
A very fine satin finish may be obtained by using a rotary fiber 
brush or rubbing by hand with a paste made of pumice and oil. 

If I he surface of the casting, after the basic polishing opera- 
tions with No. 120 and No. 180 emery, is buffed with a muslin 
or fell w heel and emery cake, a satisfactory base for a wire-brush 
satin finish is obtained. On large Hat surfaces, a fine, mild sand- 
blasl before wire brushing is quite essential to maintain a uni- 
form satin appearance. Before using the wire brush, the prepared 
surface must be thoroughly cleaned of grease and dirt with a 




• ) 



\| I MINI M I «> M I' \ N 'I OF \ M I- II U \ 



ii table solvent 01 by means of preheating for ]/fc hour at 1-00 I 

to 600° I 

\ (> inch tii.uiM i<i w 1 1 < brush made ol nil kel, stainless steel 
01 German silvei wir< f04 inch to 0.010 inch in diameter and 

; ■ ■■ it I i i |> in i" 600 i p iii i- used In i.'i\<- ihe desired 
it in appearance Vis \\\\\\ the n ulai wire brush finish, these 

I br < leaned f i equenl I > 
\ 1 1 closel) approximating I he Inn at in Imi^li can lir nli 

I Mi .i l»< i indei when surface conditions perrnil the 

( i l,i et|uipm H\ ■'■ ii bell ol \< B0 I 1M> 180 ;m<l 

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iii | * i I I. hand I i n i s hi i 1 1 1 1 |>u m in 

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MECHANICAL FINISHES 23 



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some purposes, but it can only be applied at the mill. It is ef- 
fective in hiding the structural markings of metal that is subse- 
quently to be given an Alumilite finish. 

SANDBLASTING 

Sandblasting is a rapid and inexpensive method for finishing 
aluminum. It gives a uniform matte surface, appropriate for 
some articles. Bui the plain, sandblasted surface, because of its 
oughness, will in time collect and retain dirt. Therefore, in some 



! 



case-, it may be desirable to protect the sandblasted surface by 
a lacquer or a clear varnish coating, making it easy to maintain 
ihe finish by washing. Likewise, the sandblasted surface may 
be given an Alumilile finish, if desired. 

A wide range of blasting materials is available and lends dif- 
ferent color effects to the aluminum surface. Washed silica sand 
is probably the most commonly used abrasive, although in some 
cases steel shot, pulverized silicon, carborundum sand or other 
materials may be employed. Silica sand and steel shot impart a 
light gray color; pulverized silicon, a light blue cast; and car- 
borundum sand, a dark gray. Because metal abrasives leave par- 
ticles on the aluminum surface that form discoloring products of 
oxidation, they are not favored. The use of four grades of silica 
sand is des< ribed in detail below. 
I The coarse sandblast finish is obtained by using crushed 



silica rock of 6 mesh to 20 mesh. Since it breaks down slowly 
during I he blasting, new sand should be added to keep it at the 
| required ^rit to insure uniform finish. The sand is used with a 



3/g" x 3" or Yi' x 3" chill-cast-iron nozzle, with air pressures of 
from 30 pounds to 90 pounds per square inch. For small, lighl 
| work, the %-inch nozzle and 30 pounds to 90 pounds air pressure 



are employed, while for large, heavy work the J^-inch nozzle 
and 50 pounds to 90 pounds air pressure are most satisfactory. 

| The lower pressures decrease to some extent the roughness pro- 

duced by the sand. Higher pressures than those indicated should 
no! be used, because of the excessive warping produced. During 

| the blasting, the nozzle is held at from 12 inches to 20 inches from 

I 



24 



ALUMINUM COMPANY OF A M ERIC \ 



the work and at an angle of 60° to 90° with the surface. The dis- 
tance from nozzle to work depends upon whether the article has 
an irregular or plain surface, the irregular surface requiring a 

shorter distance. Motion of the nozzle in blasting should be in 
parallel lines extending the full length of the work. The sand- 
blast uozzle should be discarded when the size of the nozzle hol« 
has increased to such an extent as to change the nature of the 

blasted surface. 

The <:rade of sand used for the medium sandblast finish is 
washed -ilica sand of from 40 mesh to 80 mesh. This -.and like- 
wise breaks down with use arid more musl be added from tim< 
to lime to replace thai which is losl through dusting and to 
keep the cutting action uniform. The nozzles, which ma> be Va 
\ 5", V s " x :>". or Yi' \ 3 ". are generally operated at air pres- 
sures of 30 pounds to 90 pounds per square inch, depending on 
the I >pe of work. Air pressures lower than 30 pound- per squan 
inch ma\ be used in some case-, hut with increased blasting 
lime. Higher pressures roughen the work excessively and cause 
imnecessar\ warping. For this type of blast, the nozzle is held 
approximately 8 inches to 1 1 inches from the surface; otherwise 
manipulation Is the same as for coarse sandblasting. 

For a line sandblast finish, a silica -and baving a fineness 
from 100 mesh to 200 mesh ma> be used. In order i<> provide a 
uniform flow, it has been found necessary to add to each loo 
pounds ol lim -and. LOO pounds of somewhat coarser -and. [f 
this is oo1 done, th< fine sand has a tendency to clog in the nozzle 
and prevent uniform flow. The addition of this coarsei material 

has no detrimental effect. The size of nozzle used with this sand 

is either M." x .V or g" \ V at an air pr sure of 50 pounds to 
75 pounds per square inch. Other considerations are the sanu as 

(or medium sandblast . 

A sandblast finish comparable to that obtained with I he fine 

-and ma\ also be obtained with flint shot ind thai has been 

partial!) broken down with use. \ l // x -*>" east-iron nozzle i 

most satisfactory, operated at 30 pounds to fit) pounds per squar< 
inch air pressui 



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MECHANICAL FINISHES 



25 



In applying sandblast finishes to sheet, it is sometimes nec- 
essary to use lower pressures to avoid warping of the metal. For 
certain work a very fine sandblast has proved desirable. In such 
dses, the blasting material is 200 mesh or finer. The nozzle is 
Yi' x 3" and the nozzle-to-work distance is 8 inches to 12 inches. 
The air pressure is approximately 45 pounds per square inch. 

If the sandblasted surface is to serve as a foundation for an 
Alumilite finish and a light color is desired, it should be immersed 
in a caustic solution (see "Frosted Finish," page 29); otherwise, 
the surface will have a non-uniform, dark-gray appearance. 

The sandblasting of aluminum requires much the same equip- 
ment as that used for other metals. There must be a sandblast 
chamber, which is a closed room fitted with an exhaust system 
and a hose for the blast. The room may also be equipped with 
some method of conveying the work past the operator or the 
automatic blasting machinery. The abrasive can be introduced 
into the column of compressed air by gravity, suction or by 
mixing directly under pressure. The method chosen for getting 
the abrasive to the metal depends on the purpose for which the 
metal is being prepared. The workman should be protected 
against dust by suitable appliances. 

There are four principal factors which determine the texture 
of the sandblasted surface and it is only through control of 
these factors that uniformity may be obtained. These factors 
are: (1) air pressure, (2) rate of introducing sand, of which noz- 
zle size is a factor, (3) grade of sand or abrasive, and (4) nozzle- 
to-work distance and angle. Once these conditions are selected, 
they should be adhered to carefully so that a uniform finish is 



secured 



TUMBLING AND BURNISHING 



Burnishing has no cutting action; it removes no metal, but 
instead applies pressure to the projecting points or particles and 
flattens or spreads them out. Ball burnishing produces a finish 
entirely satisfactory for many articles. It also gives a superior 







26 



\ I.I MINI M CO M P A N 1 or \ M I HI « V 



finish ;nul appearance bo man) products where I«»\n Hing pri< 
forbids employment of band labor. I lit 1 1 burnishing cuts the * -t 
t handling and is a finishing pnx thai, according to size <»f 
., i ■ allows Deration upon hundreds of separate pie< il i ti 
i m e i iiii' 

Burnishing produces a bright fairly smooth surface I \ th( 
t steel shot rubbing tins! the parts, li a rnplished 
tumblii i h«- work ! ethei wiih -i I shot in a w »<l lined 
rrel Soap id w i ar< added foi a lubri int The size <>i 
shot Lo be u I will depend upon several fa< tors Small I » . 1 1 ai 
niun e(!Vt'tiv< but nnin .1-1K l«»^i Ueav\ shot ha « l< iderio 
dan i i ni.il! ai lich i In size -111*1 shap< i 1 hf piiN 1 

ll 11 1 (m )\ I t .f vork inch st«*H ball s • pn 

fc h numorou other shapt uch as coin pii^ 
s! |m a I and d may Im* u fl foi peria! pui III 

hall n d (of bui dm -i \a »i I m 1 mi forth n luiid < \ 1*11 



, il h 1 cli I burnish well 1 In- I »a II m 1 I 



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MECHANICAL FINISHES 



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2. With a small charge, the pir.es will he in contact primarily with the 
shot, and more effi< ient humishing will be obtained. Pieces which have sharp 
,.,,. .,r threads will have a tendency to burr in burnishing, so the burnish- 
ing must b< restricted in amount to prevent a heavy burr, rounded corners 
01 damage to the thread*. 

. Parts which have been dipped will take somewhat longer to obtain a 
brighl surface than parts which have a machined surface. This results from 
tl». roughening of the surface l>> the dip and the fact that the machined sur- 
face itself is somem hai brighl . 

4. The longer the burnishing time, the brighter will be the resulting finish. 
Consequently, for part- where a very fine finish is desired, burnishing time 

ivv threi linn- longn than normal will be required. 

.-,. rVa a general rule, the total time of burnishing should average about one 
and on< half hours; thai is, one-half hour in the ln.t tumbling and approxi- 
mately one hour after rinsing and cleaning the barrel. 

No hud should be allowed in Ihe barrel as it lends to coat the 
hoi and is transferred lo the pieces, giving a dull, leaden ap- 
pearance to the work. The barrels should be made of hard wood, 
since pine or similar soft woods are not suitable for use. Steel 
shol ina\ become coated with aluminum dust, thus reducing its 
burnishing efficiency. A change of water is made during the 
burnishing operation to minimize this difficulty. However, if the 
lo .1 becomes coaled with aluminum, washing it in a causti< 
solution will remove the coating without affecting the steel. 

After burnishing, parts without recesses or threads may be 
dried and furl her brightened by tumbling in hard wood sawdusl 
for approximately 15 minutes to 20 minutes. It is necessary to 
use hard wood, as the pitch in soft wood may affect the surface 
finish of Ihe parts. Threaded parts or similar pieces should be 
dipped in benzine and washed in hot water. The excess water i^ 
then blown off, and the parts placed on a steam table to dry. 






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CHEMICAL FINISHES 



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ANOTHKR TYPE of finish used for aluminum involves the 
il us<- of various chemical treatments. These produce low-cost, 
protective and decorative finishes resistant to i orrosion, but 
not particularly resislanl to abrasion. Such finishing methods 
include the frosted finish, reflector linish, chemical etching and 
Vlrok ! finishes. 

KROSTED FINISH 



The frosted finish For aluminum has an attractive silvery ap- 
pcarance like I hat of tinel\ etched glass. It is useful for finishing 
small or intricately shaped articles not adapted to machine 
methods. Since it is not permanent, however, and fingerprint- 
ea i I > . it usually needs a further protection such as an Alumilite* 
finish. The frosted finish is used together with the Alumilite 
linish on refrigerator trays, and is likewise applicable to many 

ol her products. 

The first step in the frosting of aluminum is to etch in a hot 
caustic soda solution. This operation may be carried out in a 
wide range of conditions depending on the gauge of the metal 
and the finish required. Immersion for 1 minute in a l."> per cent 
sodium hydroxide solution at 160° F. to 180° F. is frequent 1> 
employed, although the concentration may vary from 2 per cent 
to _.") per cent, and the bath may be heated to boiling. Th* 
etching process is greatly accelerated at the boiling temperature, 
but the caustic may dry in streaks causing stains. To overcome 
this difficulty in special applications, some manufacturers use a 
5 per- rent caustic soda solution, followed immediately by im- 
mersion in a 2 per cent solution. The second solution is too weak 
to attack the surface of the aluminum in a short lime, and there- 



*i> 



Patented process 



29 



30 ALUMINUM COMPANY OF AM ERICA 



fore eliminates to some degree the problem of staining. Both the 
alkali concentration and the bath temperature must be kepi 

reasonably uniform. 

After the etching, the metal is rinsed in clear, cold water. If 
the time between these two steps is kept at a minimum, the 
caustic dries more uniformly and the danger of staining is min- 
imized. Cold water is preferable to hoi water because it, too, 
lessens discoloration. 

The third step is to immerse in strong nitric acid to neutralize' 
any sodium hydroxide left on the surface. A satisfactory solution 
is made by mixing 2 parts of concentrated nitric acid with 1 
part of water. However, concentrations as low as 10 per cent 
are sometimes used. This part of I he process removes the colored 



film left on the surface by the caustic solution and leases the 
metal with a clean, frosted finish. 

Again the article is rinsed in clear, cold water and last I > dried 
on a steam table. Extreme care should be used to prevent water- 
marks. In some cases air is used in drying. 

When aluminum alloys which contain substantia] amounts of 
silicon are to be frosted, it is necessary to add hydrofluoric acid 
to the nitric acid. One part of concentrated hydrofluoric acid to 

8 parts of concentrated nitric acid is a good proportion to use. 

For alloys 51S and 53S 1 part hydrofluoric and I parts nitric 

arid are added to 56 parts of water. The article to be finished is 

held in the caustic solution from 2 minutes to 3 minutes, washed in 
water, then dipped in nitric acid for 15 seconds to 30 seconds, 
and again washed in water and dried. The hydrofluoi i< -nil ric 
acid mixture must be u>ed cold. If it becomes too warm during 

use, it produces a yellow coating which is difficult to remove. 

In the case of allo> sheet, particularly that which lias been 

heat treated, the caustic solution does not remove the stains 

incident to manufacture. The stains can be removed, however, 

hv immersion for 1 minute in a cold solution containing I pail 
each of nitric and hydrofluoric acids in 98 parts of water. This 
is followed by a rinse in cold water and a dip in cold, strong nil ric 
acid. See table on page 32. 



i HEMICAL FINISHES 31 



DIFFUSE REFLECTOR FINISH 

An etched surface having high reflectivity and good diffusing 
quality for light can be produced chemically. For lighting fix- 
tures used indoors, plain etched reflectors are used with good 
results. However, when exposed outdoors, the accumulation of 
diil <>n the roughened surface, together with any superficial at- 
mospheric attack may cause substantial loss in reflection effi- 
ciency. 

Two different procedures are available for etching. In one a 
solution containing 5 per cent sodium hydroxide and 4 per cent 
sodium fluoride at a temperature of about 180° F. is used for the 
first step. After rinsing, the article is immersed in a cold nitric 
acid solution made up of equal parts of nitric acid and water. 
The other procedure, less widely used, consists of first etching 
the aluminum surface with a dilute solution of hydrofluoric acid 
and then treating with a cold, strong nitric acid solution. 

ETCHING 

Designs may be readily etched in an aluminum surface. Ex- 
tensive use of this process is being made in the manufacture of 
aluminum signs, name plates for motors, vacuum cleaners, etc., 
and dials for speedometers, vacuum gauges and the like. 

In this type of engraving, the areas which are not to be etched 
an' protected by means of a "resist," which will withstand the 
anion of the solution. To form this "resist" the desired design 
is printed on the aluminum surface with etching ink, the moist 
surface dusted with 200 mesh asphaltum powder, the surplus 
shaken off, and the remaining powder removed from the uninked 
areas with talcum powder. The asphaltum powder is then fused 
by heating to a temperature of 360° F. to 420° F. 

The surface of the article with the protected design is then 
activated in a 10 per cent solution of hydrofluoric acid, and 
rinsed. After that it is subjected to the action of a solution 
which etches the metal in the unprotected parts and engraves 
the design on the surface. Various etching solutions may be 
used, but the one which has found greatest favor for action on 




32 



ALUMINUM COMPANY OF AMERICA 



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CHEMICAL FINISHES 33 



aluminum contains iron chloride and hydrochloric acid. Hydro- 
chloric acid alone may be used effectively in certain cases. How- 
ever, if the aluminum is not uniformly clean, the attack of this 
acid ma> result in a streaked appearance. It is desirable, there- 
fore, to clean the surface with nitric-hydrofluoric acid. If the 
hydrochloric acid is saturated with sodium chloride, a smoother 
etch is obtained. The action may be accelerated by adding small 
amounts of cobalt or nickel chloride to the acid solution. Some- 
limes the work is dipped in stannous chloride solution (3 per 
cent to 10 per cent) at about 86° F. for a minute or so. This 
deposits tin on the aluminum surface. The article is then rinsed 
and etched as has been described. By these procedures, fine lines 
can be etched to a substantial depth without loss of definition. 

After the work has been etched to a suitable depth, it is re- 
moved and rinsed. The background may then be colored with 
black nickel, enamel, colored Aliimilite finishes, or left plain. 
The resist*' is then removed by suitable organic solvents. 

For etching, it is desirable to employ special "etching" sheet. 

ALROK* FINISHES 

A series of finishes, identified by the trade-mark "Alrok," can 
be obtained on aluminum and its alloys by chemical processes. 
These finishes are less resistant to abrasion and corrosion than 
the electrochemical finishes, but the procedures are less costly 
and require a minimum of equipment. The coatings may be used 
as such for their protective value or they may be used as a sur- 
face preparation for paint finishes. 

The Alrok finishes are either colorless, or a bluish- or greenish- 
gray. The color may not be entirely uniform because of varia- 
) tions in the metal structure. The protective value of these finishes 

is enhanced by special inhibitor treatments. They may be dyed, 
but the colors are not so brilliant as those obtained with the 



i 



| electrochemical finishes. 

\\ hile Alrok finishes are still in their infancy, they should find 
numerous applications in industry. 

) *Produced by a patented process. 



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ELECTROLYTIC OXIDE FINISHES 



• 



T 



II K OXIDE FINISHES formed on aluminum by anodic 
treatment in xarious electrolytes comprise one of the most 
important classes of finishes for aluminum. The hard corundum- 
like anodic coatings, which can be formed only on aluminum, 
are among the most durable of known finishes and give protec- 
tion and permanence to the surface. They are attractive in ap- 
pearance, have high resistance to corrosion, high dielectric 
strength, high reflectivity and good adsorption for dyes and 
mineral pigments. The properties of the coatings vary, depend- 
ing on (he composition of the alloy, the composition of the elec- 
trolyte and the temperature and voltage of operation, as well 
as the time of treatment. 



ALUIWILITE FINISH 

The Alumilile* process is the most practical of the anodic 
treatments. It is used to produce plain, dyed or mineral-pig- 
ment-colored surfaces. It comprises anodic treatment of alumi- 
num in a suitable electrolyte so as to secure a dense, adherent 
coating of aluminum oxide. The process differs from electroplat- 
ing in that the articles to be treated are attached electrically as 
anode rather I ban cathode in the electrolyte. In electroplating, 
a metal is deposited on the article being coated, while in anodic 
treatment, in effect, oxygen is deposited instead of metal and 
combines with the aluminum to form aluminum oxide, integral 
with the surface of the metal. 



Characteristics of the Finish: One of the outstanding char- 
acteristics of Alumilite finishes is high resistance to abrasion. The 
hardness of the corundum-like finish, applied under suitable con- 
ditions, may readily be observed by rubbing with steel wool or 

*Patented process for producing oxide finishes. 



35 




I enelian blinds made of alumi- 
num add to the decorative beau I j 
of litis interior. Ttie smooth, hard 
surface, resistant to corrosion and 
easily cleaned, is obtained by tin 

Alumilite process. 



lluminum windows with a plain 
Mumilite finish are attractive, 
easily cleaned, resistant to cor- 
rosion and do not stain adjoining 

surfaces. 




Housi hold art\ a\ 
made aitrcu > 4h 

dyed and ph \ luiT\ I 

lilt 



ELECTROLYTIC OXIDE FINISHES 37 



uiher abrasive material. This hardness is one of the properties 
I hat has made ihe finishes so valuable for cafeteria trays, street - 
car handrails, automobile pistons and for numerous other ap- 
plications where hard surfaces are essential. Quantitative tests 
on Alumilite finishes with an abrasive wheel have shown them 
to be among I he hardest of finishes. 

The value of these finishes is not confined to high resistance to 
abrasion; they can also be made highly resistant to corrosion. 
Plain aluminum, when allowed to stand in contact with water 
for prolonged periods, frequently suffers a superficial attack, and 
l he surface of the metal may take on a dark and stained appear- 
ance. On aluminum refrigerator trays, to name only one example, 
this discoloration is objectionable; an application of an Alumi- 
lite finish to the trays, however, provides the desired protection. 
\lumilite-finished surfaces of certain types are resistant to the 
corrosive action of moisture and salt water. 

For maximum resistance to corrosion under these conditions, 
coatings should be sealed, preferably in a chromate solution. 
The chromate sealers* give the coating a yellow color, so can be 
used only when this color is not objectionable, as for example, 
where the coating is to be painted. For many purposes, less 
severe in their requirements, other sealing processes* are avail- 
able. They are proving useful for architectural parts such as 
store fronts which must resist atmospheric attack for prolonged 
periods, and still present a pleasing white appearance. 

Some Alumilile coatings are very adsorptive. By immersing 
an Aiumilite-finished article in a suitable dye bath,* it readily 
adsorbs the color. The dye penetrates the coating, giving deep 
colors which have an attractive underlying metallic sheen. It is, 
of course, recognized that these dyed coatings, similar in this 
resperi to dye in fabrics, will eventually fade on continued ex- 
posure to Ihe weather. These colored finishes are best used for 
interior service. Cocktail shakers, inlay material for plastics, 
automatic pencils, fountain pen parts, ash trays, cigaret cases 
and compacts are some examples of their use. Two-colored fin- 

* Paten ted processes. 





*.^>wSSS 



Alumilite-finished store fronts an d entrances are economical to maintain, do n 
stain adjoining surfaces and harmonize with modern architectural rf< ign. Many 

standard moldings are available. 

Polished and Mumiiilc-Jinished escalator housings in Marshall Field and Co., 

Chicago. 



\ 



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ELECTROLYTIC OXIDE FINISHES 39 



ishev or contrast ing colored and uncolored sections, make pleasing 
• Heels for decorative work. Portions of the design are stopped 
oft" w hile the other part is being finished, as is done in the etch- 
ing process described in the preceding chapter. 

For more severe service, mineral pigment may be adsorbed 
in I he coating.* These colors are not usually as brilliant, nor do 
(hey produce the translucent effect of the dye colors, but they 
possess a greater light fastness. Such coatings are now in out- 
door servi< e for lamps on bridges, for statues and for other 
architectural applications. They retain their color satisfactorily 
for at least several years, although their ultimate life is not yet 
determined. 

A sealing treatment may be used on Alumilite finishes which 
has a considerable advantage for certain types of products. For 
example, the adsorptive characteristic of Alumilite finishes would 
be undesirable on refrigerator or cafeteria trays, because grease 
and food coming in contact with them would stain the finish. 
The sealing treatment, however, makes it non-adsorptive and 
tain proof. This treatment effects a change in the structure of 
the coating, which gives a marked increase in its resistance to 
i orrosion . 

Hard Alumilite finishes are not adaptable for certain forming 
operations, because of cracks which occur at relatively sharp 
bends. By the proper selection of the electrolyte and the coating 
conditions, however, it is possible to produce a finish that may 
be fabricated without visible cracks, but resistance to corrosion 
is sacrificed by this procedure. 

The coating of large, flat sheet surfaces presents special prob- 
lems. During anodic treatment, structural markings, traceable 
to the grain of the metal, may become prominent. This effect 
will usually be concealed by making uniform markings on the 
surface. Fluted sheet, for example, does not show these structural 
markings. Finishing with coarse steel wool, prior to anodic treat- 
ment, also reduces the streaks on certain types of sheet. 

When dirty, the surfaces of Alumilite-finished articles usually 



*i>. 



Patented processes. 



Aluminum refrigerator tra and 
evaporator doors are I by 

Alumilite process. With this finisl 

Ike trays are resistant to the corrosive 
action of tap water and the evaporator 

doors are made permanent attracti 





Aluminum automobile pistons 

I, >re and after the Mumilt 

> is appi I. The UumilUe 
trod gives tfu -ions a hard 
hut velvet-like surface highly re- 
tool to abrasion. 



\ practical use < tie I Ian 

ghowi by tfu • ■ 
aluminum oar- plates. 





ELECTROLYTIC OXIDE FINISHES 41 



an be restored to their original appearance merely by wiping or 
rubbing with a cloth. However, when grease and food stains are 
present, a^ on cafeteria trays, a more thorough cleaning is nec- 

ssarj The cleaner should be selected with care, for many alkaline 
i leaner- di»olve the coating. The fact that these cleaners may 
contain inhibitors which pre\ent attack on metallic aluminum 
does QOt insure their safe use on Alumilite finishes. 

In reviewing this discussion of properties of Alumilite finishes, 
(he i.i 1 1 standing feature is the versatility of the finish. Both ad- 
sorptive and non-adsorptive finishes may be formed at will. A 
wide variation in degree of thickness is available. All this mean> 
tha plain or colored coatings suitable for a wide variety of uses 
can be obtained with the Alumilite process. 



Color Matching of Alumilite Finish: Where a structure is 

composed of several parts, fabricated in different ways, the se- 
lection and finishing of these various parts present a definite 
problem where accurate color matching is required. 

The appearance of an Alumilite finish is affected to a large ex- 
lent by the original surface of the metal. Different effects are 
obtained on buffed, frosted, satin-finished, and sandblasted alu- 
minum. \sa general rule, the rougher surfaces appear darker after 
beins given an Alumilite finish. Very often these surface differ- 



- r- 



ences may serve to secure attractive contrasts, but. where an ac- 
eptable match is required, great care must be used to see that 
the proper surface preparation is employed before applying the 
\lumilite finish. 

Generally, plain Alumilite finishes have an attractive silvery- 
white appearance. There is, however, some variation in the color 
when different alloys and tempers are treated, since the alloy- 
ing constituents affect the color of the finish. The presence of a 
considerable content of silicon in the metal may result in a gray 
or almost black tone: chromium gives a yellowish cast. The 
amount of the alloying constituent alone, however, is not the 
only factor that affects the color: the condition of the alloying 
constituent in the metal is equally important. Among the heat- 



42 



ALUMINUM COMPANY OF AMERICA 




Many pieces oj aluminum hospital equipment, such ai trays, pans and basil 
are finished by ih<> Alumilite process. This finish provides a smooth, hard surface 

thai a ill mil chip and which is easily krfti clean anil sanitary. 



treatable allo\>. it is found that in the heal -treated tempers (he 
color of the Alumilite finish is lighter, as a rule, than when the 
metal i- not heat treated. As a result of these condition are 
must be exercised in making up assemblies of diiferenl alloys t<> 
be certain thai the shades of color will match or harmonize. In 
some instances, thi- coloring effect ma> be turned to advantage 

and in the case of certain silicon alloys al tractive dark colore 
may be obtained in the finish^. 

In architectural applications it is generalh necessary to em- 
ploy, in close proximity, parts fabricated in various ways, and 
still maintain a uniform appearance in the structure. For ex- 
truded parts. 53S alloy i> most commonly used, and generally in 
the "as-extruded" form. Castings of B21 1 alios with an Alumi- 
lite finish match the coated 53S reasonably well. The sheet to 

be u^ed in assemblies ma\ be either 2S or 3S, but preferably the 
former. \s has been discussed earlier, special preliminary finisli- 



ELECTROLYTIC OXIDE FINISHES 43 



ing operations are often employed to make the structural mark- 
in u> of I he sheet less noticeable. 

With special reference to castings, it is obvious that to obtain 
a satisfactory appearance the castings should be as dense and 
tine grained as possible; any porosity or lack of uniformity will 
be particularly pronounced in the Alum i lite finish. 

Frequently it is necessary to weld parts that are to be finished 
b\ the Mumilite process. To secure welds of satisfactory ap- 
pearance, special precautions are necessary. Where 53S is welded, 
2>> wire must be employed in place of the silicon alloy wir*> w hi< h 
might ordinarily be used. Even with this wire, the weld may be 
noticeable. It is therefore beneficial to apply a coarse-ground 
finish to the part to make the weld less apparent. For example, 
I he weld is far less noticeable on a surface ground with No. 120 
emery than where the surface is buffed. 



The Coming Process: The material to be coated comes to th* 
\lumilite-finishing room with some suitable mechanical finish, 
which is designed to give the desired appearance in the com- 
pleted article. It must first be cleaned. Organic solvents, solu- 
tions of mild alkalis or caustic soda may be used for this pur- 
pose. If a buffed surface is being cleaned, however, it is necessary 
to select the cleaning conditions that will minimize any attack 
of the aluminum. 

The work, after coming from the cleaning bath, is rinsed in 
water and treated as anode in a certain acid electrolyte. The time 
of treatment in this solution usually varies from 10 minutes to 
50 minutes. 

After the anodic treatment has been completed, the finish 
may be left as it is if staining will not be objectionable, or il 
may be sealed or colored. The sealing and coloring operations 
are simple treatments in certain chemical solutions. The coloring 
may be performed in one of two ways. Dyes may be applied by 
immersing the anodic-treated articles in the dye bath for a few 
minutes. Usually dyed finishes are sealed to prevent subsequent 
staining. To apply mineral pigments two or more dips are re- 









m 



i 














ilwninum is used extensivel 
a many types of lighting 
quipment. W hi n nisfu d b 
the ilzak proa naienled 
light-re fleet ion t •lh< ien 
high as n per cent are ob- 
tained. Illustrated are aft 
the lyj of ilzak ion 

Their on tal high i 
may Eh maintained * i en /// 

exposun -nh Dirt 

is easily washed I liar 

glasslike far* n Hitl ,*r 
no loss in reflectix ity. 




i 



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i 

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i 
i 
i 



ELECTROLYTIC OXIDE FINISHES 45 



<|uired. The mineral-pigmented finishes may or may not be 
sealed, (ienerally. the work should be wiped or buffed at the end 
• -f the coating procedure. 

The Alumilite process is easy to use and any plater can learn 
to handle it after proper instruction. As a result, it has become 
an outstanding method for finishing aluminum. The various 
steps in the process are covered by patents, and information as 
to their use may be obtained from Aluminum Company of 
America. 

ALZAK* REFLECTORS 

Alzak aluminum reflectors are characterized by a combination 
of permanence and high reflectivity not obtainable by the older 
finishing methods for aluminum reflectors. An oxide coating pro- 
vides the surface protection and the high reflection factors are 
i he result of a special electrolytic treatment** of suitable sheet 
prior to applying the anodic finish. 

The reflection efficiencies of Alzak reflectors as now made 
are about 80 per cent to 85 per cent for the specular finish, and 
75 per cent to 80 per cent for the diffuse finish. In addition to 
highly specular surfaces produced by polishing, and diffuse or 
matte surfaces produced by etching, it is possible to obtain 
gradations of surface intermediate between the two extremes. 

Alzak reflectors, properly maintained, retain their high efli- 
ciency under severe conditions of temperature and exposure, and 
the hard, glasslike coating permits cleaning methods not suit- 
able for untreated aluminum reflectors. Soap and water will re- 
move normal accumulation of dirt. When necessary, however, a 
mild abrasive or a wax emulsion can be used to restore the bril- 
liant finish. Where the reflector is exposed to the weather, ac- 
cumulations of dust and dirt should be removed by cleaning at 
regular intervals to prevent attack of the coating. 

Further information concerning the commercial availability of 
Alzak reflectors may be obtained from the nearest sales office of 
Aluminum Company of America. 

*Registered trade-mark. **Pat<Tilcd process. 



ELECTROPLATING ON ALUMINUM 



* 




ECORATIVE electroplates on aluminum, such as bright 
chromium, nickel and oxidized copper, are used primarily 
>r ornamental purposes. They add nothing to the resistance of 
aluminum to corrosion; in fact the plated material has lower re- 
sistance to corrosion than plain aluminum if there is any lack of 
continuity in the coating. Such electroplates, while they are per- 
fectly satisfactory where the corrosion conditions are not too 
severe, may not be satisfactory for continuous outdoor exposure. 
Chromium applied directly on aluminum has several desirable 
characteristics. The coatings are ornamental, have high resistance 
to abrasion and resist alkaline solutions particularly well. How- 
ever, they are not very satisfactory against atmospheric cor- 
rosion. 

Zinc electroplated on aluminum has no adverse effect on the 
metal resistance to corrosion, and has been used to improve 
tb^ electrical contact characteristics of the aluminum surface. 

ZINC PLATING 

Zim is plated on aluminum in a thin layer from a special 
cyanide solution. Plating may then be continued in any plating 
bath that is suitable for plating on zinc. Where zinc alone is the 
mrtal coating, the plating is quite resistant to corrosion, but 
when other metals such as nickel or copper are applied over the 
preliminary zinc coating, the product does not resist corrosion 
well, but is likely to blister or peel on exposure to moisture, be- 
cause of the difference in electrolytic solution potential between 
these metals and aluminum. 

Heat treatment, so that the coating will alloy itself with 
aluminum, may be used to make copper, plated over the zinc 
'"flash," more resistant to corrosion, but this procedure has the 
disadvantage of adding to the cost and in some cases may affect 
the temper of the alloy. 

47 



48 



ALUMINUM COMPANY OF AMERICA 



Plating on aluminum after a cyanide zinc 'flash/' except for 
zinc alone, can be recommended only for a Limited l\pe of service. 
Such deposits would be quite satisfactory for indoor use, and 
can be considered durable where such service conditions are 
fairly dry. 

The zinc alone, on the other hand, has been used to resist 
i ertain types of corrosion. Such a deposit has also been employed 
for radio shield cans to effect a special type of contact be I ween 
the chassis and the can. 

The plating procedure for applying the preliminary zinc la\er 
is as follows: 



1. (.'■ hi lh<' metal surface b\ a short immersion in a cleaner containing 
1 ounce to 3 ounces per gallon each of sodium carbonate and trisodium phos- 
phate. The cleaner is used at a temperature of 180° F. t«» 200 F, 



2. Rinse in clear, cold water. 



3. Treat for 1 minute in .5 per cenl hydrofluoric acid at room temperature 
The etching ef t of this solution appears necessary for good adhesion of the 

plate. 



•i. Rinse in clear, eold water. 



5. Apply a thin coating h> plating from 1 minute to H) minutes a1 a current 
density of 1 ampere to 5 amperes per square foot, in tin- following bath: 

Zinc cyanide <Zn (CN)*) 4 ounces per gallon 

Sodium cyanide (NaCN 4 oun< per gallon 

\mmoniurn hydroxide Mholh I sp. ^r. 0.90) i ounces pei gallon 

Peptone ... ' v ounce per gallon 

6. The plating ma> then be continued in solutions suitable for plating on 
zin< such as cyanide-copper, nickel or acid-zinc. 



If zinc alone is the coating metal, it is usually best to continue 
Ihe plating in an acid hath because of l lie greater speed of plat- 
ing, and because the acid hath does not attack (Ik aluminum 
through the coating. 




ELECTROPLATING ON ALUMINUM 49 



CHROMIUM APPLIED DIRECTLY ON ALUMINUM 

1 in chromium plates can be readily applied directly to alu- 
minum from an ordinary chromium bath at about twice the 
usual current density used for plating over nickel. For cleaning, 
a shoii treatment in the alkaline cleaner, already mentioned for 
zinc plating, followed by immersion in hydrofluoric acid, is all 
thai is required. A slight attack of the aluminum appears nec- 
essar\ in securing an adherent deposit. With certain of the cast- 
ing alloys, an additional treatment in nitric and sulfuric acids, 
i >r nitric and hydrofluoric acids, is required to whiten the surface. 
The chromium invariably comes from the bath with a dull gray 
color. When plated from a hot solution (118° F.), the chromium 
is lighter in color than when plated from a cold solution (80° F.), 
but the deposit from the cold solution is much more readily 
polished to a high luster. This finish is not quite equal in bright- 
ness to ( hromium over nickel, but has numerous applications. 
Not all chromium polishing compounds are equally satisfactory 
for polishing this finish. Tests of the materials of the various 
manufacturers will indicate the most suitable compound. 



NICKEL PLATING 

A preliminary nickel plate may be employed as a foundation 
tor any other plate. Chromium applied over buffed nickel is 
brighter and more attractive than when it is applied directly to 
t he aluminum. 

The nickel plate should be applied to a specially roughened 
aluminum surface. It is not sufficient simply to produce an ir- 
regular surface such as might result from sandblasting. It is 
necessary to produce a roughened surface in which the irregu- 
larities and pits are undercut into the aluminum, so that the 
coating electrodeposited in these undercut openings will be satis- 
factory keyed or anchored to the surface. A structure of this 
character is produced with special etching reagents. The reagent 
dissolves certain portions of the surface selectively, and the 
number, character and placing of these openings are greatly af- 






HOT FUDG 

i SU NDAES 




Various methods of finish ing. in- 
cluding black nickel plating, 
plain etching and coloring by the 
\lumilite process, are employed 
in the manufacture of aluminum 

name plates. 



*&£&&&* 




Chromium applied over nickel- 
plated aluminum gii perma- 
nent non-tarnishable fin ish fo 

these utensils. 



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52 



ALU M I.NUM CO M l'ANY F A M K H I ( \ 



Manganese Solution — For aluminum alios sheet and other 



wrought materials 

I [ydrochli u n id 1 1( I sp. gr. Lis 

\\ ater 

Manganese sulfate (M11SO4.2H2O 

Temperature 

Time ■ ^ 1 real menl appi 1 iximate) . . 



1 gallon 

-'.h gallon 
ounce 
»0 I'. 

\ .'> .I i> I - to .'JO sen hi<I-n 



\ cid N>lu1 ion —For easl ings 

Nitric a< id II \< > s|i -I 1. 1.; 
Hydrofluoric a< id III- t8' 
Tem| >ei -Mure 
Time <»f 11 ea1 not ut: 
Die C a s ti n g€ 
. Permanent -Mold ( as1 ings 

SandCastii 









:', pan - 

i pai 1 

U) mi F. 



I .', seconds 1 ' > 'Ml si < •< tridw 

30 i 'in I- 1 1 1 90 onds 

00 se< oilds 1 I .''I stN onds 



Thi time oi treatment in each of these solutions must be car* 
full} determined !>> experiment, as this is the most important 
step in the plating procedure. This is accomplished i>\ plating a 
si ip- .1 s|> imens etched foi different length <>i time and then 
bending 01 breaking them. \ good * 1 « j ** »*- i l does not flake off 
n im I < 'i this treatment If the time of etching is too short, the <l< 
posit 1 1 not adhere, while if ii is too lonj the deposit is rough 

ml cannot l»< readih bulled. 
In the etching treatment the two most important factoi to 
control are the temperature and the acidity. Since even sti<jhi 

han in Lhe temperature cause ;i considerable change in ihc 
time which is required for best results, temperature should In* 
held instant '<i the time of treatment should be varied i<> coj 
rect foi temperature < han . The acid it) should !><• kept a< 
near!) instant as possible. Where a solution 1- being used con 
limi.ill more frequent replacement <»l ih< acid consumed in 

Lchii the aluminum a r< nee< ary. 

I h' jaii of metal being treated 1- also a factor lhai affect 
the etching 1 [me. I bin sections are a< led upon more rapid I) 1 han 
thick ones, probabl) be* 1 the) tend to heat rn re 1 dily, 
ihrre i a smallei w< - hi of metal lo absorb the I il oi i< 

• 1 inn Hard-rolled sheet II > requires great < in etcl 

ing 1 han s< »f t metal. 



ELECTROPLATING ON ALUMINUM 53 



Occasionally metal will come from the manganese etch with a 
lark film on I he surface caused by the alloying constituents. 
This can be brushed off or removed by treating in concentrated 
nitric acid or a nitric and sulfuric acid mixture. 

There are also differences among the alloys; 51S being the 
most readily plated. 17S-T alloy is readily plated in the form of 
crew-machine products, but often gives trouble in sheet form 
because of metal streaks. 

The containers for the acid etching reagents should be lead- 
lined and should also be painted with a mixture of 1 part bees- 
wax to 4 parts paraffin. This is particularly necessary above the 
solution line, where the attack of the lead takes place. 

6. Rinse: The metal is rinsed in clear, cold water. A double 
rinse is desirable for keeping acid out of the plating bath. 

7. Nickel Plating: Various nickel baths may be used for plating 
aluminum. A bath which has been used considerably in the past 
is as follows: 

Nickel sulfate (NiS0 4 .7H 2 0) 19 ounces per gallon 

Magnesium sulfate (MgS0 4 .7H 2 0) 10 ounces per gallon 

Ammonium chloride (NH 4 C1) 2 ounces per gallon 

Boric acid (H 3 B0 3 ) 2 ounces per gallon 

Temperature 90° F. to 95° F. 

Current density 15 amperes per square foot 

pH (Colorimetric) 5.8-6.0 

More recently the following bath has been used with excellent 
res u Its : 

Nickel sulfate I NiSO^HjO) 45 ounce- per gallon 

Nickel chloride (NiCl 2 .6H 2 0) 2.28 ounces per gallon 

Boric acid ( H3BO3) 4 ounces per gallon 

Current density 30 amperes per square foot 

pH (Potent 1 u metric) 2.0-2.5 

Temperature 140° F. to 150° F. 

To overcome a tendency toward pitting, frequent additions of 
hydrogen peroxide are necessary (0.053 ounce of 30 per cenl 
solution of hydrogen peroxide per gallon of electrolyte; the hy- 
drogen peroxide should be diluted with water before adding to 




54 



ALUMINUM COMPANY OF A M E R I C \ 



the bath). An excess should be avoided, for it results in embrittle- 
menl of the deposited nickel and reduces throwing power. 

The deposited nickel should be smooth and velvety in ap- 
pearance, and readily buffed to a high luster. The usual lime 
buffing compositions may be used. 

Heat treatment of nickel-plated aluminum parts affords a dis- 
tinct improvement in adhesion of the nickel deposit. For the 
common wrought alloys, a heal treatment of 6 hours at 300° 1 

is suggested. 

For a deposit on aluminum that must stand moderate outdoor ser- 
vice, a thickness of a t least 0.001 inch is recommended. Generally, 
the thicker the deposit, tin* more resistanl to corrosion it will be. 



Other Metals on Aluminum: As has been mentioned ear- 
lier, the nickel deposit may serve as the foundation for a variety 
of other plates. Certain observations are warranted in connec- 
tion with the metals mosl commonly plated. 

Chromium may be readily applied over nickel from the ordi- 
nary chromium baths now being used commercially. Experience 
indicates that a deposit of about one and one-half minutes is 
advisable. Longer plating times ma> raise a few tin> blisters on 
a small percentage of the plated articL particularly if the nickel 
is thin. Tests have indicated that these articles are satisfactory 

in resistance 1 to corrosion. 

Copper may also be readih applied over the nickel from 
either a cyanide or an acid bath. Generally, the acid bath is 
better, for it lias less tendency to undermine the plating. 

Bra>^ ma> be applied to t he nickel, but because of t he tendency 
for some cyanide baths to undermine the plating, particularly 

ood nickel plating LS needed. 

Silver, as well as numerous other metals. ma\ be readih ap- 

plied to 1 he nickel. 

Colored or oxidized finishes ma\ be obtained on these deposits 

by the usual procedure In a few instance where the coloring 
solution is strongh alkaline, it i^ advisable to reduce the e ength 

of the alkali. 



ELECTROPLATING ON ALUMINUM 






Black Nickel: Black nickel plating may be applied directly 
to aluminum Such a finish is commonly used on the etched 
background of aluminum name plates. The coating is sufficiently 
durable for interior service, but is not of much value outdoors. 
A typical black nickel solution which may be employed on 
aluminum is: 

Nickel numonium sulfate (NiS0 4 . (NH 4 ) 2 S0 4 .6H 2 0) 8 ounces per gallon 

Zinc sulfate (ZnSG 4 .7H 2 0) 1 ounce per gallon 

Sodium sulfocyanate (NaCNS) 2 ounces per gallon 

Nickel anodes are used in the solution. The voltage is aboul 
1 volt, and I .be current density 1 ampere to 2 amperes per square 
fo<H . The solution is kept nearly neutral by the use of zinc car- 
bonate. 

TESTS OF DEPOSITS 

It cannot be o^ eremphasized that the successful plating of 
aluminum can only be accomplished when regular and severe 
tests of the quality of the plate are made at frequent intervals. 
This is necessary because it is so easy to obtain deposits on alu- 
minum which have a nice appearance, but, nevertheless, are of 
little value. Regular checks on the adhesion and resistance to 
corrosion of the coatings should be made at frequent intervals. 











I 




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I 



ALCLAD PRODUCTS 



• 




LCLAD" is a registered trade-mark used to identify duplex- 
metal products sold by Aluminum Company of America 
which are unusually resistant to corrosion. 

These products are made by bonding a layer of high-purity 
metal, or a special aluminum alloy, to the surface of a base of 
aluminum or aluminum alloy, until it becomes an integral part 
thereof. The thickness of this surface layer is carefully selected 
mi that the final product will provide satisfactory resistance to 
corrosion and retain substantially the physical characteristics of 
the basii metal. While the tensile and yield strengths are some- 
what lower in this material, this may be compensated for, if 
necessary, by using a slightly thicker gauge of sheet. 

Alclad products are so durable that they have withstood the 
corrosive action of 5 years' exposure to salt spray in the labora- 
tory without loss of mechanical properties. The protection given 
by the coating is also sufficient to prevent, by electrolytic action, 
corrosion of the base alloy at the sheared edges of the sheet as 
veil as other sections that have become exposed by scratching. 
There is, however, some solution of the pure aluminum surface 
layer at these broken or exposed areas. The corrosion spots are 
only superficial and can be readily removed without destroying 

the protecting surface. 

This product can be in the form of aluminum sheet and plate, 
and is extensively used for airplane parts. Where the sheet must 
meet the most severe service requirements, it may be further 
protected by painting or even by anodic treatment followed b> 

painting. 

There are advantages, for some applications, in substituting 
certain aluminum alloys for the high-purity aluminum in the 
surface coating. This change is made when increased abrasion 
resistance is required as well as good resistance to corrosion. 



57 




I'UYI LACQ1 IK 




i:\amel i iM-ni - 



* 



\HE [ >|. o| painl or lacquer finishes on aluminum ma} b< 
Found desirable or ( en i i ary for certain applications In 

,,,.. m i in painl i n titute ibr mo i ecoi mi< al i > pe of 
.1. iral ion <\ iu« h i an be emplo I, while in other ■ ii 
, i, H ,1,1. i tit the standpoint of k ction, espe< i >IK wh< i 
, .,,, condition? of exposure are encountered. Tl lintin ol 
iluminum follows the &enei il technique u ed for the paint ii 

I .1 ll< I IIM I il 



\\ |,, i, i ,. .mi or lai [uer coatin i to be employed iolel> I 

s i \ hi 1 1- p< i ial preps i ion ". eded. 'I I 
i o| 1,1 e I i lean and IV from grease it itisfactoi 
i,, | , |„ obtained < Him ;« thoi >u b cleanii with >lvenl 

uffii , i \\ Ii n painl m. i dow fi i protecl ion, ho\* i 
, r pi iuI I- n must be taken jusl is in tl i ' • h 
metal* Adequate preparation of the surfa qu J impoi ml 
i | i be selei 1 1-" of ■■ al i i m ■!.»!■> prim I Afferent alloj 
ilun mum beh i • om< bal differently with ' th< 

ibilil ) to liol'l painl bul in all cases aluminum alii 
in pi t< i w ii li painl I Ii in ii • mi >sl <>i her metals. 

II I M I Ol \i i m I OMPOSi i i< »n 

1 1, ( . [ y 8 tS, IS, S2S md 5 IS bav< I n found t< mo* 
what bettei painl adhesion than >«i< h ;ill<>\ is ITS 24S 
s uul .1 S VIclad I > i'1'l U "I - IS beha^ in m I be 



u ne ,,, , i 2S il-Mii.ii g I I the pu minum 



i mi h«> ii I* I I In' in- til 

Vlosl of i be painl in., w In* Ii i for otecl i irp 

appli n iln- beat treal ibU- <i * J. If i ! Mention is .• en 
i. ni prepai ii i<»n imd the »!-• in >it» x | m< no 

P V\ Hill '('♦'< 



dillh nil vim it in_ li'ipial i pri 'I 'iil'l 1 

Pests ba^ e ahow n thai . e> en \\ In « rel t : i\»'l\ \ 
i , |. ition is employed, i?ood results ma) I I. 







I 




Serviceable finishes offering beauty and high resistance to corro m arc important 
in certain types of aircraft* It is common practice to give each part an aruodu 
oxide coating followed by a coat of zinc-chromale primer before ass< mbly; aim nun an 

paint may be used as a top coat. 



A unique and efficient painting system is employed in the finishing oj Gn Mind 
buses. Each part to be painted is suspended on a conveyor^ sprayed h h a cfiemical 
cleaner, rinsed, neutralized and dried by heat. \ pigmented primer containing 
zinc chromale is then spra I on. dried and baked. The compleh a mbly 
nished trith a surfacer and enamel <>r lacquer in accordance with automotive 

practice. 




i I \ T , L\M)UER AND E N A \1 I L 



61 



SURFACE PREPAR 1TION 

Ther( are man> methods available for preparing an aluminum 
mi, if Cor [tainting Surface roughening by sandblasting oi 
scratchbrushing, although sometimes employed, is ordinarily 
no1 nece n and oftentimes is hikIimi d>le, particularly in I h < 
i of 1 Inn sheet ections Surface roughening tends to impair I h 
natural oxide film which Is always presenl on aluminum and thi 
renderi the metal more susceptible to con ion when moist up 
penetrate the paint coating. Except under ver> >\» ial condi- 
tion therefore, surface rou^henin^, particularly sandblasting, 
is not re< ommended. 

rhe limplesl form of surface preparation i- 3olven1 cleaning. 
This can he accomplished by washing with doihs saturated 
with solvent, or- h> actual immersion in the solvent, followed by 
wiping with clean cloths. In either case ii i-> difficult, on a corn- 
men id tie, to secure an absolutely clean surface in ihis man- 

1/ 

ner since a thin film of grease usually remains on ihe surface as 
ih» solvent evaporates I n certain cases, this film ma> be re- 

ved hv heating. \n improved form ofsolvenl cleaning is that 

of solvenl vapor decreasing, which has proved entirely adequate 
for maii\ applications. This overcomes many of the objection- 
ible features of ordinary solvenl cleaning hni mere cleaning is 
not iillici'Mii for the more severe conditions of exposure. 

\ number of chemical treal ments ha\< appeared on the market 
during the pasl lew >ears There are two aeral types— mild 
alkaline solutions of the -odium phosphate or -Milium silicate 
type, and dilute .aid solutions containing phosphoric acid. The 
alkaline solution^ appear to (lean the surface "i the aluminum 
withoul serioush attacking il and are fairly effective in securing 
?ood painl adherence. The phosphoric acid solutions, which 
usually . ontain water, alcohol or another organic solvent, in ad- 
dition to phosphoric acid, are much more effectiv< Vppannth 
lh<\\ form a thin layer of aluminum ph»»>phate on (he surface 
which is quite insoluble and which tends to protect the aluminum 

tins! corrosion. There is a variety of proprietary treatments 
of i bis i \ pe a\ ailahle ea> ihe market . 







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I T , LACQUER AND ENAMEL 






Vftei iriing or otherwise treating i he surface, the metal 
hould b< thoroughly dried h\ exposure lo air or by heating 
before paint can be applied. The mHal should receive a mini- 
mum amounl handling after cleaning. The election of th 
i urface preparation from the foregoing methods is de 
rmined h\ the met;il lo be painted, I In- rharai ter of the struc- 



ture and th( ervice conditions to which ii will be exposed. 



\ll I MODS Ol M'IM.M \ I ION 



Hi' methods of applying painl finishes ire not peculiar to 
aluminum. Brushing dipping and spraying ma> b< employi I. 
hi lition application b roller coa i i r i i particularly advan- 

la peciallj where the metal is to be subsequ ttly forn I 

into v.ii li i|» i in the case of bottle caps. Manufacturing 

con iderations 111 determine the selection of th< besl method 

m| l|)| I lull 

PRIMING PAIN I S 

Whili il is not nec< .u> to employ ;m\ special paintin prac- 
tice !■ 'i ippl ii' de rat i\ e i atmj to aluminum a i i full) 

eli ted | -.nil i ni: rstem must he employed for protective < it- 
in: I l<< value oi 1 1 1«- system will depend to a I xtent on 

th( el i in □ if the proper primer. The primer should bav< - 
resist to n isture penel ra1 ion i<» pre^ al surfa tcti< n. 

hould adhere well, form a od b ise i >i second • i 

should I llli in mini, it j\ i> pig nirill 

<H (In ii h. us primer pigments which bav< I d for 

duminum, zinc i oromate alone appears to p pronounced 

ii niliiliiii\ «• properties. \ prime i pigmented entirely « b 

zinc chroma te bas been found, in mosl instance i<> give th< 

besl i' ili> fi iIk- standpoint <>f protection Su< h primi ai 

immercialh available. 

I r l« > severe conditions of exp«^ tha i lai iri v 



of primers which will doubtless p satisfactory. In m 

cases iluminuni [tain i bas been found i< > be a ver> sal i 

primer It fulfills iln* requirement of high impermeability to 




This spotoscope {right) and the 
binoculars (below) illustrate the use 
of pigmented coalings over an Alu- 
m it He finish. A lum il He fin ishes form 
excellent bases for paint and similar 
coatings on aluminum because of 
their inert or non-reactive character- 
istics. 




A pleasing finish d ilionc 

enamel and polished alum am 

is shown I, 




I 




PAINT, L \ < o U E It AND E N \ M E L 



- 



ii n< ■•! adhesion to the metal, and pre ots aufficienl 

ucceedin^ inais. Where finishing coats oi la< ruer 



tool It t« ,i 

e to be employed, a primei should be selected which is par- 

ul.ii <l< «l for i heir use. 

Propei ele< tion of the vehicle is likewise ne< > ai ■ in< it i 

in importanl factor in determining I In- moistureproofing prop- 

1 1 H of the i i.i in i . \\ 1 1 1 1 i he alum i m nn primer, ;» long oil varnish 

vehicle oi i vnthetic resin varnish vehicle is suii ihi Zinc 

hromate in a synthetic resin varnish base such a the alk><! 

m in oi phenolii i e in i \ pe is • •v rii.-ni 

FINISHING. COATS 

Once the primer is prop<Tl> applied almosl an) durable ex 



l« rioi painl or enamel ma) be employed for ihe finishing "| . 
1 1- I h< p include oil ba e paints, long oil varnish* md en im< 

/nthetii i in finishes, and aluminum painl made either with 
long nil varni h or with a fnthetic resin varnish Wh ih 
color oi aluminum painl i atisfactory, ii is i p iall) recom 
mended I a use of its properties ol great opa it) '""I high im 
|m inn tbilit) t< moisture. I h< synthetic resin en tmels hav< iIm 
l>< i found to be ••thvtive as finishing coats h m f their 

lurabilil ■ 

Bituminous paints ma) also Im 1 used for finishing iluminum, 
hut pvh i h< ) ii •• ii -' 'I. a primei i >i tin um< matt i iaJ is n 
ii. • nd i It the black color of the hi luminous paint is objection- 

il>l< ii m.i\ In* piu'mi'iited witli aluminum powdei to in 

ill i tive finish 

Where extremel) fasl air-drying finishes are required, p> 
roxvlin riuers ma\ be i mploved I Ii ma\ be set i I in m\ 

I 1 

desired color, including iluminum. The aluminum-pigm< 
lac(]tiei ir< perhaps, the mosl durabl sin the tin) flal 
. •iiiiniiini prote the nitrocellulose from the destine th i n 

i null lii I he im. .-i durable I are th< m linii . 

cert .uii im< .im! i >i j) ni hel i< i on In u _ i i m, f h 

primei musl be i urefull) sele* I. as iousl) poinh ut, • 

thai ii will be entirel) compatible with the Ii finish in 







Collapsible tubes litho- 
graphed after fabrication 








■ 



iluminum foil is attroA 
lively decorated 63 printing 
lithographing <tmt lacquer- 
ing. 




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* 



PAINT, LACQUER AND ENAMEL 



67 



coats. The manufacturer of the lacquer is in a position to make 
re< mmendations for suitable primers. 



CLEAR FINISHES 

Tran parenl coalings may be employed to preserve the natural 
appearance of aluminum or to maintain a polished finish during 
outdoor exposure. They may be either lacquers or varnishes. For 
durability the lacquers musl be of the type containing appreci- 
able amounts of Imhl-resisling resins such as alkyd, vinyl and 
methacrylate resins or containing cellulose esters such as cel- 
lulose a< •< -lubutyrate. Some \arnishes made with ghcerol-phlha- 
late re us ha\ e given good service in such applications but most 

arnish< are of little use because of their tendency to turn 

.Mow . 

VITREOUS ENAMEL 

( ertain types of vitreous enamels may be applied to alumi- 
num bul at pi ent they have only limited application. This is 
becai the relatively low melting point of aluminum (about 
I 00° F.) restricts the enamel to low melting compositions. Tin 
finely ground enamel is mixed with turpentine or water and 
brushed or sprayed in a uniform coaling onto the aluminum, 
which is then heated to temperatures from 950° F. to 1050° F. 
Bes1 i- mils are obtained at the higher temperatures, but with 
gi i risk of distortion of the metal. Most of the enamels which 
may be applied to aluminum are not entirely resistant to slov 
a( I ack \>\ water and hence are unsuited for many types of service. 



s 




ALUMINUM COMPANY OF AMERICA 



SALES OFFICES 



ALBANY, N. Y 90 State St. 

ATLANTA, GA 1818 Rhodes-Haverty Bid 



£• 



BOSTON, MASS 20 Providence St.. Park Square 

BUFFALO, N. Y 1880 Elmwood Ave. 

CHARLOTTE, N. C 619 Johnston Bldg. 

CHICAGO, ILL 520 North Michigan Ave. 

CINCINNATI, OHIO 16th Floor, Times-Star Bldg. 

CLEVELAND. OHIO 2210 Harvard Ave. 

DALLAS, TEXAS 1601 Mien Bldg. 

DAVENPORT. IOWA 919 Kahl Bldg. 

DENVER, COLO 63-1 U. S. National Bank Bldg. 

DETROIT, MICH 331 1 Dunn Rd- 

FAIRFIELD, CONN Boston Posl Rd. 

HARTFORD, CONN Capitol Bldg., 410 Asylum St. 

INDIANAPOLIS, IND 1008 Merchants Bank Bldg. 



KANSAS CITY, MO 2306 Power & Light Bldg. 

LOS ANGELES. CALIF 5151 Magnolia Ave. 

MILWAUK EE. WIS 735 North \\ ater St . 

MINNEAPOLIS, MINN 1060 Northwestern Bank Bldg. 

NEWARK. N. J 1111 Academy Bldg. 

NEW ORLEANS, LA 1512 American Rank Bid* 

NEW YOR K. N . Y 230 Park \ > - 

PHILADELPHIA, PA 2307 Fidelil v-Phila. Trusi Bid* 

PITTSBURGH, P\ Gulf Bldg. 

ST. LOUIS, MO 1000 Continental Bldg. 

709 Rialto Bldg. 



SAN FRANCISCO, CALIF 



SEATTLE. WASH L005 While Bldg. 

TOLEDO. OHIO 915 Ohio Bank Bldg. 

W VSHINGTON, D. C 605 Southern Bldg. 



ALCOA 




nd the adjac* re n Un- 

applied to tin du- uxnirmm ( Ai w] 

:hnical s1 i >st rig ontrol 

lion of Ale i Aluminum , . . from t i, 

r< I I le ] " luci uniform inu 

aluminum alloys, in brm. 



ALCOA 



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PRO. Ml V 

EDO* PfiFSS COPPORATIO* 
PITTSBURGH, P; 



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